Pathway toward Scalable Energy-Efficient Li-Mediated Ammonia Synthesis

Area of Science:

• Electrochemistry
• Materials Science
• Chemical Engineering

Background:

• Lithium-mediated ammonia synthesis (LiMAS) is an emerging electrochemical route for ammonia (NH₃) production.
• The process involves Li⁺ electrodeposition, Li nitridation, and Li₃N protolysis, with Li⁺ electrodeposition being a critical step requiring current oscillations for solid-electrolyte interface (SEI) stability.

Purpose of the Study:

• To investigate the key parameters influencing LiMAS performance, including nitrogen (N₂) pressure, proton donor, and lithium (Li) salt properties.
• To optimize LiMAS for high ammonia faradaic efficiency (FE) and current density.
• To evaluate the technoeconomic feasibility of high-pressure LiMAS compared to existing ammonia production methods.

Main Methods:

• Electrochemical synthesis with controlled current oscillations during Li⁺ electrodeposition.
• Systematic variation of N₂ pressure, proton donor type (e.g., alcohols), and Li salt counterions (e.g., BF₄^-).
• Analysis of NH₃ selectivity, Faradaic efficiency (FE), current density, and energy efficiency.
• Technoeconomic analysis of the LiMAS process under various conditions.

Main Results:

• Ammonia FE increased with N₂ pressure up to 20 bar; proton availability became limiting at higher pressures.
• 1-butanol as a proton donor and larger anions like BF₄^- in the Li salt enhanced NH₃ FE and SEI stability.
• A peak NH₃ FE of ~70% and a current density of ~-100 mA/cm² were achieved.
• LiMAS utilizing H₂O has a theoretical maximum energy efficiency of 27.8%, significantly influenced by the proton source.
• High-pressure LiMAS demonstrated superior technoeconomic feasibility compared to ambient LiMAS and modified Haber-Bosch processes.

Conclusions:

• Optimizing N₂ pressure, proton donor, and Li salt is crucial for efficient LiMAS.
• High-pressure LiMAS is a promising, cost-effective technology for sustainable green ammonia production.
• LiMAS has the potential to revolutionize large-scale ammonia synthesis, offering a sustainable alternative to conventional methods.